The multi-faceted intricacies of the optimal decentralized control problem (even for linear dynamics) are widely recognized in the control literature. With the hope for the existence of any convenient (let alone convex) parameterizations dispelled (see, for example, references [1]}, [2]}), recent research advances have resorted to modern convexification or regularization methods [3]}, [4]}, [5]}, [6]}, [7]}. In this context, the so-called System Level Synthesis (SLS) methods from [8]}, [9]} provided an original and insightful perspective on distributed controller design, by exploiting the classical work from [10]}. The connections between the SLS and classical parameterizations of stabilizing controllers have been further elaborated in [11]}, [12]} and, more recently, in [13]}. However, the SLS framework: (a) necessitates implementations which communicate internal states (i.e., controller's or plant's states), producing Transfer Function Matrices (TFM) of the controller's representation with dimensions equal to that of the plant's state vector, while (b) allowing the scalable, specialized implementations from [9]} only for stable plants.
